Sodium perborate is obtained commercially by addition of hydrogen peroxide to a sodium metaborate solution at a temperature close to 20.degree. C. according to the continuous or batch fabrication methods. It is a peroxide of the formula: ##STR1##
To obtain a good yield in relation to the hydrogen peroxide and a chemically stable perborate, synthesis is performed in the presence of magnesium salt. It is known that the Mg.sup.++ cation is a good stabilizing agent of active peroxide oxygen in an alkaline medium; but this additive has an undesirable action on the crystallization of sodium perborate. The magnesium cation modifies the crystalline aspect of the sodium perborate by favoring the formation of platelets of increasingly small thickness as a function of the increase of the Mg.sup.++ concentration. These platelets agglomerate and build up in very fragile, disordered structures. Consequently, the physical characteristics of the resultant sodium perborate are not those corresponding to a valuable commercial product for which market demand is now oriented, i.e. dense and mechanically resistant crystals are not produced.